1. Field of the Invention
The present invention relates to a method and apparatus for mixing a polyester resin continuously.
2. Description of the Related Art
Polyester resins are superior not only in such mechanical properties as tensile strength and impact resistance but also in heat resistance and weathering resistance, so they are applied to various uses. Particularly, polyethylene terephthalate resin (PET resin) is superior in the above properties and are light-weight in comparison with glass, so is utilized widely in the manufacture of food containers, etc.
For obtaining a high-quality product from PET resin it is necessary that the PET resin used should possess good moldability and physical properties. To this end, a certain degree or higher intrinsic viscosity value (IV value) is required. Especially, a reproduced PET resin is markedly lowered in its IV value due to, for example, the heat applied thereto during molding. Therefore, also from the standpoint of upgrading a reproduced PET resin product it is required to develop a method capable of thickening a PET resin efficiently.
However, polyester resins have a property such that their IV values are lowered by a hydrolysis by water produced on melting, thus leading to deterioration in subsequent molding characteristics of resin products.
For avoiding such an inconvenience there has been developed a continuous polyester resin mixing apparatus provided with a chamber having a material feed port and a material discharge port, mixing rotors inserted rotatably into the chamber, a vacuum pump for dehydrating the interior of the chamber under reduced pressure, a material feed means connected to the material feed port, and a chemical agent feed means connected to the chamber for adding into the chamber a chemical agent (a chain-extending agent) for increasing the viscosity of a feed resin or for suppressing a decrease of the viscosity.
According to the above continuous mixing apparatus, a polyester resin with a chain-extending agent added thereto is mixed within the chamber while being subjected to dehydration under reduced pressure, whereby the lowering of IV value can be prevented even without pre-drying the resin (see Japanese Patent Laid Open No. 8-183083, which is hereby fully incorporated by reference).
According to the above prior art, however, since a chain-extending agent is added before dehydration is performed under reduced pressure through vent openings, that is, before the feed resin begins to melt, there occur both a chain-extending reaction and a hydrolysis reaction of the feed resin simultaneously at the time of melting of the resin.
Consequently, the molecular weight of the feed resin lowers in the hydrolysis reaction during melting of the resin due to water originally contained in the resin, thus giving rise to the drawback that the degree of increase of IV value based on the chain-extending reaction becomes smaller.
It may be a countermeasure to increase the amount of the chain-extending agent to be added, taking the water originally contained in the feed resin into account. However, particularly in case of reproduced PET flakes, the amount of water is as large as several thousand ppm, so the method of improving the thickening effect by only increasing the amount of the chain-extending agent requires using a large amount of the chain-extending agent, which is uneconomical.
On the other hand, in case of a PET resin it is known that if a feed resin is dried beforehand up to a water content of not more than 50 ppm and thereafter an aromatic tetracarboxylic acid dianhydride is added, followed by mixing and melting, even reproduced or off-grade PET resins of low viscosity can be utilized effectively as foamed products (see, for example, Japanese Patent Laid Open No. 8-231750, which is hereby fully incorporated by reference).
For drying the feed resin beforehand up to the aforesaid water content it is usually required to use a batch type dryer of a large scale, thus resulting in increase of the equipment cost. Besides, in a batch type drying process the feed resin cannot be treated in a continuous manner and therefore it is necessary for the feed resin to be stored in a large quantity after drying, thus leading to an increase of the storage cost.
The present invention has been accomplished in view of the above-mentioned circumstances and it is the first object of the invention to enhance the thickening effect in a thickening process applied to a feed resin beforehand without subjecting the resin to drying in a dryer.
It is the second object of the present invention to permit both drying process and thickening process for the feed resin to be carried out continuously in case of subjecting the feed resin beforehand to drying in a dryer and thereafter to thickening and thereby reduce the feed resin storage cost.
For achieving the foregoing first object the present invention has adopted the following technical means.
According to the first aspect of the present invention, in the foregoing apparatus for mixing a polyester resin continuously, a portion of the chamber located upstream of the chemical agent feed means is used as a drying section in which the feed resin before the addition of a chemical agent thereto is subjected to mixing while being dehydrated under reduced pressure, and a portion of the chamber located downstream of the chemical agent feed means is used a thickening section in which the feed resin after the addition of the chemical agent thereto is subjected to mixing while being deaerated under reduced pressure.
Thus, in a single chamber which is continuous axially, the feed resin not containing a chemical agent (a chain-extending agent) for thickening the feed resin or for suppressing a lowering in viscosity of the feed resin is once subjected to mixing while being dehydrated under reduced pressure in the drying section and thereafter the feed resin containing the chemical agent is subjected to mixing while being deaerated under reduced pressure in the thickening section. Therefore, most of the water originally contained in the feed resin is removed in the drying section.
Thus, since the chemical agent is added in the thickening section after the water originally contained in the feed resin has been removed in the drying section, the thickening reaction is prevented from being impeded by the water originally contained in the feed resin, so that not only it is possible to increase the IV value in the thickening reaction to a greater extent than in the prior art, but also the amount of the chemical agent added can be kept to a minimum.
More specifically, given that the inside diameter of the chamber is D, an axial length of the drying section is L1 and that of the thickening section is L2, an axial length of the chamber is set so that L1/D is in the range of 15 to 25 and L2/D is in the range of 15 to 30.
The above L1/D range in the drying section is set as a length sufficient for drying feed resins which vary in the content of water. For example, in case of PET resin it is known that there occurs a thickening reaction if the water content lowers to near 50 ppm. Such a lowering of the water content can be attained by setting the L1/D ratio at 15 or more and thereby ensuring a sufficient residence time.
On the other hand, if the water content lowers to 50 ppm or thereabouts, a further dehydrating effect cannot be expected even if the drying section is made very long. Besides, the water resulting from melting of the feed resin is dehydrated under reduced pressure in the subsequent thickening section. Therefore, it is sufficient for the L1/D ratio in the drying section to be not more than 25.
A predetermined residence time is also required for improving the IV value of a polyester resin. The longer the thickening section, the longer the residence time and the larger the amount of resin capable of being treated. But in view of the strength of the mixing rotors it is impossible to make the thickening section very long. Further, from various experimental results it has turned out that a residence time of at least two minutes or so is needed in the thickening section for effecting the thickening reaction to a satisfactory extent. For these reasons there was established the above L2/D range in the thickening section.
A residence time of two minutes or more of the material to be mixed in the thickening section can be attained by providing a mixing portion having retentivity in the mixing rotor portion positioned in the thickening section. The mixing portion can be attained by a rotor segment having a feed blade and a return blade or a kneading block having reverse flights.
The adoption of such a mixing portion is desirable because a sufficient residence time is obtained at a short L/D ratio and because a compact equipment results. Further, although the material to be mixed increases its temperature by being mixed and melted and tends to be deteriorated, but its deterioration caused by an excess rise in temperature is suppressed effectively in the kneading portion constituted particularly by a rotor segment, which is desirable.
For achieving the foregoing second object the present invention has adopted the following technical means.
According to the second aspect of the present invention, in the foregoing apparatus for mixing a polyester resin continuously, the material feed means is provided with a dryer for dehydrating the feed resin, a storage hopper connected directly to the dryer and adapted to store the feed resin after drying in a moisture-proof state, and a shutter means capable of being opened and closed and disposed between the dryer and the storage hopper.
According to this apparatus, since the storage hopper is directly connected to the dryer and a shutter means is interposed therebetween, if a predetermined amount of the feed resin after dried in the dryer is fed into the storage hopper by opening the shutter means, followed by closing the shutter means and feed of the next predetermined amount of the feed resin into the dryer, the next predetermined amount of the feed resin can be dried while the preceding predetermined amount of the feed resin present in the storage hopper is subjected to a thickening process, and thus in the case where the feed resin is subjected to pre-drying and subsequent thickening, the feed resin drying process and thickening process can be carried out in a continuous manner.
In the apparatus according to the second aspect, moreover, since the feed resin is subjected to pre-drying by a dryer separate from the chamber, it is not necessary to set the L/D value of the chamber so long as in the apparatus according to the first aspect and hence it is possible to adopt a mixer whose L/D ratio is within the conventional range.
In the apparatus being considered, the dryer is not specially limited if only it has a function of drying the feed resin so as to be 50 ppm or less in water content. For example, there may be adopted a hot-air dryer in which hot air is circulated for drying or a dehumidifying dryer in which a dehumidified inert gas is circulated for drying.